<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    .box {
      width: 0px;
      height: 0px;
      /* border-top: 10px solid red; */
      border-bottom: 10px solid green;
      border-left: 10px solid transparent;
      border-right: 10px solid transparent;
    }
  </style>
</head>

<body>
  <div class="box">

  </div>
</body>

</html>